new chick has third foot.

I picked up my Plymouth rock chicks today from a local feed store and when I got home I realized that one of chicks had a third foot coming out of the upper leg. Its toes on one of the other feet curled right over and could not walk around. I put it in the brooder with my one week old chicks and they immediately started pecking at the extra foot. Is this rare or does it happen more than you would think? I did call the feed store and swapped out the chick for another.

I didn't get a pic of the chick. I had to hurry up and get back to the feed store before they closed. I have been a fan of the site for quite a while but never joined. I am hooked, I have six production reds, one mix, and now four aracauna , two light brahma and 3 plymouth rock chicks. Need a bigger coop.